1. Field of the Invention
Embodiments of the present invention generally relate to tuner device systems and methods, and, in specific embodiments, to tuner device systems and methods that allows for pitch detection that maintains pitch error accuracy and response time while allowing for the frequency of a clock signal to be reduced.
2. Related Art
In a conventional pitch detection device (e.g., a tuning device that detects the pitch of a musical note that has been input and indicates the deviation from a standard pitch), the pitch of the musical note that was input (pitch of the measured signal) is detected using a clock signal of several hundred KHz to several MHz. Here, the method of pitch detection of a measured signal in a conventional pitch detection device is explained with reference to FIG. 7.
The conventional pitch detection includes a central processing unit (CPU) that regards one clock of a clock signal as the smallest base unit (the smallest base unit that becomes the standard at the time of the measurement of time length) and measures the duration of one cycle of the measured signal (S71) a predetermined number (fixed number) of times. Then, the average time (average cycle) of the duration of one cycle of the measured signal is calculated (S72). From there, the pitch is found (S73).
With respect to the conventional pitch detection device, in order to effectively reduce the pitch error accuracy of the measured signal (error that occurs at the time of pitch calculation) and the response time (the time from when the measured signal is input until the pitch of the measured signal is newly detected), a clock signal of one hundred kHz or more (e.g., 500 kHz or more) is necessary.
When a clock signal of 500 kHz or more is used, the current consumption by the pitch detection device is about one hundred μA due to the height of this frequency. It is well known that lowering the frequency of the clock signal is an effective method to lower the current consumption of the device (e.g., to several tens of μA). However, with such devices, when the frequency of the clock signal is lowered, the pitch error accuracy of the measured signal and the response time at values are not properly maintained.
As noted above, the conventional pitch detection device regards one clock of a clock signal as the smallest base unit (the smallest base unit that becomes the standard at the time of the measurement of time length) and measures the duration of one cycle of the measured signal generated between the interrupts based on zero-cross. Next, one cycle of the measured signal is detected (as known in the art) based on an auto-correlation function. Then, finally, the duration of 1 cycle detected is determined by multiplying the time length of a clock signal with the number of counted clocks.
Using the conventional pitch detection device causes a measurement error k to arise, as will be explained. First, when one cycle of the measure signal was the portion between the k−1 clock portion and the k clock portion, the measurement error with respect to the duration of one cycle of the measured signal, in the k clock portion, amounts to error due to the fact of being a little longer than the true duration of one cycle of the measured signal. Second, when one cycle of the measured signal was the portion between the k clock portion and the k+1 clock portion, the measurement error with respect to the duration of one cycle of the measured signal, in the k clock portion, amounts to error due to the fact of being a little shorter than the true duration of one cycle of the measured signal. As such, the true duration of 1 cycle of that measured signal is a duration in the middle of the total duration of the k−1 clock portion and the total duration of the k+1 portion of a clock signal. However, in a pitch detection device, the duration of one cycle of a measured signal is measured as the duration of the k clock portion of the clock signal. As such, because the measurement of the duration of one cycle of the measured signal is discretely carried out with one clock of the clock signal as the smallest base unit, the errors that occur in that measurement also occur discretely.
The pitch error accuracy and the response time of the measured signal can be measured using formulas (1), (2), and (3) where the pitch error accuracy is d (cent), response time is t (s), the frequency of a measured signal is f (Hz), the number of times of detection of a cycle of a measured signal is N (times), the frequency of the clock signal is Fc (Hz), and the number of clock signals counted within the total duration that measures 1 cycle of a measured signal N times is p (clock). Furthermore, as for pitch error accuracy d, error is indicated in cent units (1 cent is an interval of 1/1200 of an octave).d=1200×log 2(p/(p−1))  (1)t=N/f  (2)p=Fc×(N/f)  (3)
Furthermore, Fc/f is the number of clocks counted during 1 cycle of the measured signal. Accordingly, the number of clocks p indicated by formula (3) can also be varied as in the following numerical formula (3′) as the number of clocks counted within the time the cycles of a measured signal have been measured N times.p=(Fc/f)×N  (3′)
The p represented in this numerical formula (3′) is the total number of clocks counted during the total duration in which the cycles of the measured signal (e.g., the music notes collected by a microphone) were measured N times.
Furthermore, in numerical formula (1), the logarithm of the ratio of the number of clocks p and the number of clocks (p−1) is calculated. In other words, the pitch error accuracy d indicates an error that occurs between the number of clocks p and the number of clocks (p−1).
Accordingly, the pitch error accuracy indicating the error that occurs between the number of clocks p and the number of clocks (p+1) can be found as shown in numerical formulary (1′).d=1200×log 2((p+1)/p)  (1′)
However, this numerical formula results in reduced pitch error accuracy. For this reason, hereinafter, the pitch error accuracy d will be found using numerical formula (1).